How much of a problem is it if a road bike is too small?

Have just picked up a second hand road bike that is a compact frame. It may however be slightly too small - not catastrophically, but slightly. I'm thinking layback post and longer stem should sort it out nicely, particularly as I like small/chuckable feeling bikes. Is there any flaw in my thinking or problems I'm not seeing?

How much are we talking? If you end up with a huge stack of spacers under the stem it might look a bit odd but should be ok. If you manage to get it to fit it will be fine, otherwise you'll know pretty soon if you need to replace it.

Slightly too small's better than slightly too big. If it's a whole size too small it could be an issue, but other than that most people buying a bike end up tweaking fit with a couple of spacers, a couple of cm on the stem or a wee slide back or forward on the saddle rails.

For mountain bikes, sure. For road bikes though? How much do you think you will 'chuck' it around? Unless you're riding Road Bike Party (in which case you're allowed a small-ish bike). The thing with road bikes is that you don't really change position much (compared with MTBs), so if it's not a good fit it'll hurt.

You don't want a set back seatpost. Fit your position relative to teh BB then worry about reach. A long stem is no issue. As mentioned, most of the pros ride small frames with long stems to get a lower position with more weight over the front wheel. I ride a 120mm stem on my track bike without any issues. 150 is pushing it a bit. For height rather than handling.

I'll also bet that the "150" is too long and you need something smaller like a 120. Post a picture with you on it.